1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus for transmitting a pilot in a multi-antenna system.
2. Related Art
With the popularization of information communication services and the advent of a variety of multimedia services and high-quality services, demands for communication services are rapidly increased. To satisfy these demands, various wireless communication techniques are studied in various fields.
Next-generation wireless communication systems are required to transmit high-quality large-capacity multimedia data at a high speed by using limited frequency resources. To achieve this in a wireless channel with a limited bandwidth, it is required to overcome inter-symbol interference and frequency selective fading which are generated when multimedia data is transmitted at a high speed while maximizing spectral efficiency. Orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) techniques are spotlighted among techniques developed to maximize spectral efficiency.
OFDM uses a plurality of orthogonal subcarriers and orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter performs IFFT on data and transmits the inverse-fast-Fourier-transformed data to a receiver. The receiver performs FFT on the received data to restore the original data. That is, IFFT is used to combine multiple subcarriers and FFT is used to split multiple subcarriers. According to OFDM, complexity of the receiver in a frequency selective fading environment of a broadband channel can be reduced and spectral efficiency can be increased through selective scheduling in a spectral domain by using different channel characteristics of subcarriers. Orthogonal frequency division multiple access (OFDMA) is a multiple-access method based on OFDM. The OFDMA can respectively allocate different subcarriers to multiple users to improve the efficiency of radio resources.
The MIMO technique can be used for two purposes. Firstly, the MIMO technique can be used for the purpose of increasing a diversity gain to reduce performance deterioration caused by a fading environment of a channel. Secondly, the MIMO system can be used for the purpose of increasing a data transfer rate in the same frequency band. The MIMO technique can transmit a large quantity of data without increasing a frequency bandwidth as compared to a single-input single-output system using a single transmitting/receiving antenna.
An MIMO channel provided according to multiple antennas can be decomposed into independent channels. When the number of transmission antennas is Nt and the number of receiving antennas is Nr, the number of independent channels, Ni, corresponds to min {Nt, Nr} and each independent channel can be referred to as a spatial layer. Generally, a rank is defined as the number of non-zero eighenvalues in an MIMO channel matrix.
In a MIMO system, a transmitter performs preceding that multiplexes a transmission signal by a weight. Precoding is a technique that pre-processes a transmission signal by using a weight and transmits the pre-processed transmission signal. The weight is selected or calculated based on a channel to which the transmission signal will be subjected. In a frequency division duplex (FDD) system, generally, a receiver determines the weight and transmits the weight to a transmitter and the transmitter determines a weight to be actually used to transmit signals based on the weight received from the receiver. In a time division duplex (TDD) system, a transmitter determines the weight through a sounding channel.
A model of a received signal vector y is represented according to Equation 1.y=HWS+N  [Equation 1]
Here, H represents a channel matrix, W represents a weight, S denotes a signal vector, and N denotes a noise vector. The weight W can have a matrix or vector form. A common pilot is used to estimate the channel matrix H. A precoded pilot is used to estimate HW. That is, a receiver uses HW acquired according to the precoded pilot to detect S.
The precoded pilot can directly detect S by using the equivalent channel HW. When the precoded pilot is used in rank 1 transmission, a signal-to-noise ratio (SNR) gain can be expected in signal estimation, and thus reliability of channel estimation performance can be improved. When the precoded pilot is used in transmission of rank 2 or higher, improvement of yield as well as the SNR gain can be expected in channel estimation.
However, a multi-antenna system has a very complicated channel environment and the SNR gain and yield improvement are not secured for all the complicated channels of the multi-antenna system because there is a limitation in achieving a SNR gain and yield improvement for data streams subjected to a very poor channel among data streams input to a preceding unit and respectively transmitted to a receiver through different channels.
Accordingly, a method and apparatus for transmitting a precoded pilot to improve channel estimation performance is required.